Dstatcom Using PSB

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IECO NO l: The 27th Annual Conference of the IEEE Industrial Electronics Society

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Modeling and Simulation of a Distribution STATCOM

usin g Sirnulinks Power System Blockset

Pierre Giroux Gilbert Sybille Hoang Le-Huy2

Laboratoire Simulation de reseaux

Dept. of Electrical and Com puter Engineering

UniversitC Lava1

QuCbec, Q C, C anada

lehuy@gel.

ulaval.ca

Institut de recherche dHydro-Quebec (IREQ)

Varennes, QC, Canad a

giroux.pierre@ireq. a sybille.gilbert@ireq. ca

Feeder

PWM Inverter

Abstract

- This paper presen ts a study on the modeling of a STAT-

11. DESCRIPTION OF TH E D-STATCOM OPERATION

COM (Static Synchronous Com pensator) used for reactive power

compensation on a distribution network. The power circuits of the

D-STATCOM and the distribution network a re modeled by spe-

cific blocks from the P ower System Blockset while the control

sys-

tem is modeled by Simulink blocks. Static and dynamic

performance of a

E

Mv ar D-STATCOM on a 25-kV network is

evaluated. An average modeling approach is proposed to sim-

plify the

PWM

inverter op eration and to accelerate the simulation

for control parameters adjusting purpose. Simulation perfor-

mance obtained with both modeling approaches are presented

and compared.

I . IN TRO D U C TIO N

Electricity suppliers ar e. now aday s concerned about the

quality of the power delivered to customers. With the develop-

ments

of

power electronics, several solutions have been pro-

posed to compensate for the fluctuations observed

on

the

distribution

networks

in order to

ensure highest possible pow er

quality for the customers

[ 2 ] .

These Power Quality Devices (PQ Devices) are power

electronic converters connected in parallel or in series with the

lines and the operation is controlled by a digital controller [13,

[ 2 ] , [3],

[4]. The interaction between the PQ device and the

network is preferably studied by simulation. The modeling of

these complex systems that contain both power circuits and

control systems can be do ne on different bases, depending on

the trade-offs that we are ready to accept and on the degree of

accuracy of what we want to study (switching in power con-

verter or controller tuning). The modeling abstraction degree in

these systems can be thus adapted to the study requirements.

In this paper, two approaches to model a distribution STAT-

CO M (Static Synchrono us Com pensator) are considered and

evaluated, that is device modeling and average modeling.

Both modeling approaches take advantage of Simulink and

Pow er System Blockset to implement in the same diagram the

power circuit and control system. The models are described

and the simulation results are presented. They will be then

compared.

In distribution networks, the STATCOM (Static Synchro-

nous C ompens ator) is a shunt device that regulates the system

voltage by absorbing or generating reactive power.

Fig.

1

shows a simplified diagram of a STATCOM con-

nected to a typical distribution network represented by an

equivalent network.

25

kV

100 MVA

4

Fig. 1 Simplified diagram

of a D-STATCOM

connected

to

a

distribution

network.

The STATCOM consists mainly of a PWM inverter con-

nected to the network through a transformer. The dc link volt-

age is provided by capacitor C which is charged with power

taken from the network. The control sys tem ensures the regula-

tion

of

the bus voltage and the dc link voltage.

The D-STATCOM function is to regulate the bus voltage by

absorbing or generating reactive pow er to the network, l ike a

thyristor static compensator. This reactive power transfer is

done through the leakage reactance of the coupling transformer

by using a secondary voltage in phase with the primary voltage

(network side). This voltage is provided by a voltage-source

PWM inverter. The D-STATCOM operation is i l lustrated by

the phasor diagrams show n in Fig.

2.

When the secondary volt-

age (V,) is lower than the bus voltage

(VB),

the D-STATCOM

acts l ike an inductance absorbing reactive pow er from the bus.

When the secondary voltage (V,) is higher than the bus volt-

age (VB), the D-STATCOM acts l ike a capacitor generating

reactive power to the bus. In steady state, d ue to inverter losses

the bus voltage always leads the inverter voltage by a small

angle to supply a small active power.

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IECONOI

:

The 27th Annual Conference of the IEEE Industrial E lectronics Society

(b)

Fig. 2 D-STATCOM operation

a) Inductive operation,

b)

Capacitive operation

The STATCOM has several advantages as compared to

conventional Static Var Com pensa tor (SVC) using thyristors.

It is faster, can produce reactive pow er at low voltage, does not

require thyristor-controlled reactors (TCR) or thyristor-

switched capacitors (TSC), and does not produce low order

harmonics.

111. MOD ELING T HE D -STATC OM USING THE

SIMULINKS POWER SYSTEM BLOCKSET

As seen above, a D-STATCOM is a power electronic system

with a complex control system. Modeling the D-STATCOM

including the power network and its controller in Simulink

environment requires electric blocks from the Pow er System

Ts=1/60/360/8

Blockset [SI and control blocks from S imulin k library. We con-

sider here a +3Mvar D-STATCOM connec ted to a 25-kV distri-

bution network.

Figure 3 shows a Simulink diagram which represents the D-

STATCOM and the distribution network.

The feeding network is represented by a T hevenin equiva-

lent (bus B l ) followed by a 21-km feeder which is modeled by

a pi-equivalent circuit connected to bus B2. At this bus, a 3-

MW load is connected. A 25-kVl600V transformer and a 1

M W variable load are connected to bus B2 by a 2-km feeder.

Th e D-STATCOM output is couple d in parallel with the net-

work through a step-up 2.5125-kV A-Y transformer. The pri-

mary of this transformer is fed by a voltage-source PWM

inverter consisting of two IGBT bridges. A filter bank

is

used

at the inverter output to abs orb harmonics. A 10000pF capaci-

tor is used as dc voltage source for the inverter.

A PWM pulse generator with a carrier frequency of

1.68

kHz is used to control both IGBT bridges. The modulation

schem e used is of sinusoidal type.

Th e controller diagram is shown in Fig. 4 t consists of sev-

eral subsys tems : a phase-locked loop (PLL), two measurement

systems, a current regulation loop, a voltage regulation loop,

and a d c link voltage regulator.

The P LL is synchronized to the fundam ental of the trans-

former primary voltage to provide the synchronous reference

(sinwt and coswt) required by the abc-qd transformation. The

measurement blocks Vmes and Imes compute the d-axis

and q-axis com ponen ts of the voltages and currents.

The inner current regulation loop consists of two propor-

tional-integral ( PI) controllers that control the d-axis and q-axis

currents. The controllers outputs are the voltage direct-axis and

D-STATCOM

25kV,

I- 3Mvar

DataAWuisi lan

Fig.

3

Simulink diagram representing the D STATCOM and the d istribution network.

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IECON'O1: The

27th Annual Conference of the IEEE Industrial Electronics

Society

f-

abc

pu)

W ModeOper

S K C O S

Discrete

3-phase

P LL

ld lq dlq

VdVq -

Vabc

1,

Vabc

-

Iq.Ref

Idlq-Rsf

Unit

f

LL

Fig.4 D-STATCOM

control

system

J

\ \

4

quadrature-axis com ponents (V, and Vs) that the PW M

inverter has to generate. Th e V, and V, voltages are converted

into phas e vol tage s V,, V,, V, which are used to syn thes ize the

PWM voltages.

The network bus voltage is regulated by a PI controller

which produces the I, reference for current controller. The I,

reference comes from the dc link voltage regulator which

maintains the DC link voltage constant.

IV.

SIMU LA TIN G TH E D -STA TCO M O PERA TIO N

The Simulink diagram described above has been used to

simulate the operation of the D-STATCOM under different

conditions to il lustrate i ts static and dynam ic performance. The

simulation was done using a discrete step time (T, =

5.8 ps .

Figs.

5

and

6

show the waveforms obtained during a com-

plex test in which the dynamic response of D-STATCOM to

step changes in source voltages is observed.

The

PSB Pro g. Source

block is used to modu late the internal

voltage of the 25-kV so urce.

At starting, the source voltage is such that the D-STATCOM

is inactive. It does not absorb nor provide reactive power to the

network. At t =

0.125 s,

the source voltage is increased by

6%.

The D-STATCOM compensates for this voltage increase by

absorbing reactive pow er from the network (Q = +2.7 Mvar).

At t = 0.2 s, the source voltage is decreased by 6% f rom the

value corresponding to Q = 0. Then the D-STATCOM must

generate reactive po wer to maintain a

1

pu voltage (Q changes

from +2.7 Mvar to -2.8 Mvar).

VdVq m.Ph8

012

14

0 1 6

1 8

0 2 0 2 2 02 4

m a ?I)

Fig. Waveforms illustrating the D-STATCOM dynamic performance.

Note that when the D-STATCOM changes from inductive

to

capacitive operation, the inverter modulation index m is

increased from

0.48

to

0.87

which corresponds to a propor-

tional increase in inverter voltage. Reversing of reactive power

flow is very fast (about one cycle) as shown in Fig. 6 .

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08-9/01/ 10.00 (C)2001

IEEE 992


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IECON'Ol:

The 27th Annual Conference of the IEEE Industrial Electronics Society

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0.17 0.18 0.19 0.2 0.21 0.22 0.23

3000r

2000

- 1000

2 0

>

3 -1000

-2000

-3oooL

0 1 7 0 1 8 0 1 9 0 2 0 2 1 0 2 2 0 2 3

Time ( 1

Fig. 6 Voltage and current waveforms during the change from inductive

to capacitive operation

at

t = 0.2

s.

V. AVERAGE MODELING T O ACCELERATE

THE SIMULATION

The above simulation uses a detailed model of the inverter

that includes the sw itching of the inverter pow er switches. This

mod el requires a very small comp uting time step to well repre-

sent the P WM waveforms (T,

=

5.8 ps). The simulation time is

thus fairly long. If we are not interested to represent th e chop-

ping of the PWM waveforms, we can use instead a voltage

source having the same average value computed upon a chop-

ping period (U168 0 in this case). By using this average

model , we can simulate the system operation with a larger

step time resulting in a simulation time reduction.

Th e average model can be built based on the energy con-

servation principle. As shown in Figure

7,

the instantaneous

power must be the same on the DC side and the AC side of the

inverter (assuming an ideal inverter):

V d c I d c = va ia

+

Vbib + vc ic

1)

Th e DC current in the DC-link capacitor can be then com-

puted from the measured AC instantaneous power and the DC-

link voltage v d c as:

vaia + Vbib vcic

'dc

'dc =

(m x V d c ) L p h i

Pdc

=

v I PaC

=

V a l a + V b l b

.t

vclC

dc dc

vai,

+

vbib

+

vcic

dc

dc = 'ac __ *dc =

(b)

Fig.

7

Average modeling technique

(a) Detailed model

(PWM),

(b) Average model.

Fig. 8 shows the Simulink implementation of the inverter's

average model. On the AC side, the inverter is modeled as

three controlled voltage sources which are determ ined by three

voltages Vabc from the control system. On the DC side, it is

modeled by the D

ink

model. In this model, a capacito r (rep-

resented by an integrator) is charged by a DC current source

Average Model

l a b

I

h

DC-link-model

_ n

Fig.

8

Simulink diagram o f the PWM inverter average mode l.

0-7803-7 108-9/0 1/ 10.00 C)200 IEEE

993


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IECONOI : The 27th Annual Conference of the IEEE Industrial E lectronics Soc iety

5 2 4 0 0

P

with value computed as show n in (2). A

Switch

block is used to

avoid a division by zero at starting when the capacitor has no

charge.

The s ame dyn amic test with the detailed m odel has been

applied to the D-STATCOM average model using a t ime step

size 8 times larger than for the detailed model. The simulation

time

is

thus reduced approximately by 8.

Fig. 9 show s a compariso n between wavefo rms obtained

with average and detailed models for the case where the system

changes fro m inductive to capacitive operation at t = 0.2 s. The

waveforms shown are the D-STATCOM phase A voltage and

current , and the q-axis current I, The waveforms a re qui te

identical for both models except for the inverter output voltage

waveforms. In the detailed model, we can observe the chop-

ping of the dc voltage while in the average model, only the

average value

is

shown . We can also note that the dynam ics

of

the currents is preserved by the average model.

-

1

0.18 0.185 019 0.195 0 2 0.205 0.21 0.215 0.22 0.225 0 23

/

o

2000

10

0.18 0.185 0.19 0.195 0 2 0.205 0.21 0.215 0.22 0.225 0.23

Fig. 9 Comparison

between

responses of detailed and average models

for a step change in the network internal voltage.

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(C)200

1

IEEE

VI. CONCLUSION

A detailed model of a D-STATCOM has been developed for

use in Sim ulink environmen t with the P ower System Blockset.

Models of both power circuit and control system have been

implemented in the same Simulink diagram allowing smooth

simulation. Two modeling approaches (device and average

modeling) have been presented and applied to the case of a

+3M var D-STATCOM connected to a 25-kV distribution net-

work. The obtained simulation results have demonstrated the

validity of the developed models. Average modeling allows a

faster simulation which is well suited to controller tuning pur-

poses.

VII . REFERENCES

[11 K.K . Sen, STATC OM: Theory, Modeling, Applications,

in IEEE PES

999

Winter Meeting Proceedings,

pp. 1177-

1183.

[2]

Flexible AC Transmission Systems FACTS ),

edited by

Y.H.

Song and A.T. Johns, The Institution

of

Electrical

Engineers, London , UK, 1999.

[3] K.V. Patil , et al ., Application of STATCOM fo r Dampin g

Torsional Oscillations in Series Compensated AC Sys-

tems, IEEE Trans. on Energy Conversion, Vol. 13, No.

3,Sept. 1998, pp.237-243.

[4] C.D. Schauder,

H.

Mehta, Vector Analysis and Control of

Advanced Static VAR Compensators,

IEE Proceedings-

[SI

Power System Blockset For

se

with Sirnulink,

Users

Guide, The MathW orks Inc., 2000.

C, Vol. 14 0, NO .4, July 1993, pp. 299-306.

994

Dstatcom Using PSB

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